In the forthcoming evolution of present mobile cellular standards like GSM and Wideband Code Division Multiple Access (WCDMA), Orthogonal Frequency Multiple Access (OFDM) will be used for transmission. OFDM promises higher data rates and a more efficient usage of limited bandwidth resources than the presently employed techniques.
Furthermore, to migrate smoothly from the existing cellular systems to new high-capacity, high-data-rate systems in existing radio spectrum, the new systems have to be able to operate with flexible communication channel bandwidths. One such new flexible cellular communication system is called Third Generation Long Term Evolution (3G LTE), which is standardized by the Third Generation Partnership Project (3GPP). The 3G LTE specification can be seen as an evolution of the 3G Wideband Code Division Multiple Access (WCDMA) standard also promulgated by 3GPP.
The 3G LTE system will use OFDM as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UE) and will be able to operate on bandwidths ranging from 1.4 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported for the largest bandwidth channels. Besides high-data-rate services, 3G LTE systems are expected to provide low-data-rate services, such as speech. Since 3G LTE is designed for packet data according to the familiar transmission control protocol/internet protocol (TCP/IP), it is expected that the service that carries speech will use voice-over-IP (VoIP).
To allow an LTE terminal to connect to a suitable serving cell, a mobility function needs to be implemented in an efficient manner. As radio conditions change, e.g. because the user of the terminal moves to a different location, the mobile station needs to be handed over from one serving cell to another.
It is also worth noting that LTE will be deployed gradually in a successive fashion by the mobile operators. Therefore Inter Radio Access Technology (IRAT) mobility from e.g. GSM/WCDMA to LTE will be another important functionality. Mobility from a GSM/WCDMA system to an LTE system is obviously just one of many examples of IRAT mobility.
Cell search procedures and efficient signal power measurements are of major importance to allow the mobile station to find and connect to a suitable cell. At the same time it is important to keep the power consumption down in the mobile station, which makes constant measurement of signal strength a less attractive alternative. Therefore some type of schedule for performing measurements needs to be implemented, so that the mobile station may power down its receiver chain for as much time as possible while still being able to provide updated data for handover decisions and channel estimation to fulfill sufficient performance requirements.